Traditional drilling operations employ the circulation of a weighted drilling fluid (i.e. mud) such that the hydrostatic pressure of the drilling fluid contained in a well bore is equal to or greater than the pressure exerted by the formation being drilled. Traditional drilling can be preferable because the weight of the mud column prevents flammable hydrocarbons from entering the well bore. However, traditional drilling also creates operational challenges due to a positive pressure differential between the well bore and the formation. Examples of operational challenges include differential sticking of the drill pipe, reservoir damage due to filter cake, increased costs of well completion, and reduced permeability and production from the formation.
In response to these challenges, a drilling method called under balanced drilling has been developed. In under balanced drilling, the hydrostatic pressure of the drilling fluid is less than the pore pressure of the formation. Under balanced drilling has the potential to be hazardous because oil and gas could blowout from the well bore, releasing a large amount of flammable hydrocarbons into the atmosphere. However, improvements in blowout prevention equipment have made it possible to drill safely in an under balanced condition. Benefits of under balanced drilling include increasing the drilling rate, limiting lost circulation, limiting reservoir damage, reducing differential sticking, and reducing the cost of well completions. Under balanced drilling can be beneficial when drilling directional and horizontal wells that target oil and gas reservoirs for production purposes.
To accomplish under balanced drilling conditions, the weight of the drilling fluid must be reduced so that the hydrostatic pressure within the well bore is less than that of the formation. Standard methods of reducing weight of the drilling fluid include replacing the drilling fluid with a gas (i.e. air, nitrogen or natural gas drilling), infusing the drilling fluid with a gas to reduce the density of the mud (i.e. gas-cut mud drilling), and creating a foam from a gas and a liquid and using the foam as the drilling fluid (i.e. foam drilling). In each case, under balanced drilling involves the introduction of a gas into the well bore. The gases that are used for drilling may also be used during post-drilling operations for various well-completion and production activities such as cleaning out well bores, cleaning pipelines, and reservoir injection to stimulate production in secondary recovery projects. Secondary recovery is an oilfield term used to describe any process such as the injection of gas into a reservoir to restore oil production from a subsurface formation where the primary drive mechanism and reservoir pressure have been depleted. Pipeline cleaning, also known as pigging, is the process of forcing a device called a pig that is made of hard rubber, plastic or metal and shaped like a sphere or a cylinder through a pipeline to remove condensate that collects in low places in the pipeline.
For both under balanced drilling and other gas-related oilfield applications, the traditional options for gas selection include air (79% N2 and 21% O2), carbon dioxide (CO2), natural gas, and nitrogen (N2). The oxygen in the air presents the risk of down hole fire or explosion because the oxygen can promote an explosive atmosphere when mixed with hydrocarbons within the well bore. The resulting well bore fire can be very costly and disruptive to drilling operations. Use of air or carbon dioxide also presents the risk of increased corrosion of down hole pipe and equipment, requiring expensive corrosion prohibition and treatment to the drilling or production equipment. The use of natural gas can be prohibitively expensive for sustained drilling operations and increases the risk of hazardous exposure for drilling personnel.
Of the available gas options, nitrogen provides the most benefit for under balanced drilling while presenting the fewest associated risks. Nitrogen is inert and does not create a risk of down hole fires or explosions. Nitrogen is not corrosive and does not require additional corrosion protection for the drilling or production equipment. Nitrogen is also considered relatively safe to use, as it is not flammable and does not present an undue safety risk for personnel involved in the drilling operation. Therefore, it is highly desirable to have a supply of pure nitrogen available for use during under balanced drilling, secondary recovery projects and other oilfield operations and such as pipeline pigging. If the nitrogen is generated at the well site, producing field or pipeline site, the generation of nitrogen should be cost effective in that it does not place an undue financial burden on the under balanced drilling project, the secondary recovery operation, or pipeline cleaning process.
The nitrogen producing equipment must meet other demands that are unique to drilling, production, and pipeline operations. The physical location of the drilling operation, secondary recovery project, or pipeline access point can be remote, so the nitrogen production equipment must be able to be transported to remote places. Drilling operations typically last less than three months, so the nitrogen producing equipment must be mobile enough to move from one location to another along with the drilling equipment. The nitrogen producing equipment must also be priced such that the cost of the nitrogen producing equipment does not prohibit the use of nitrogen at the well site. Thus, a need exists for a relatively inexpensive method for producing nitrogen in which the nitrogen producing equipment can be frequently moved to remote locations.
There are four generally understood methods for generating nitrogen. The first generally understood method for generating nitrogen is cryogenic distillation. Cryogenic distillation is a process in which air is condensed into a liquid form, and then separated into component streams in a distillation column. Cryogenic distillation can produce extremely pure streams of nitrogen and oxygen. Unfortunately, the cryogenic distillation process is very expensive and is generally considered cost prohibitive for drilling uses.
The second generally understood method for generating nitrogen is pressure swing adsorption (PSA). PSA is a process in which air is confined in a chamber with an adsorption catalyst and drastic and/or rapid changes in the pressure of the gas causes one type of molecule, oxygen, to adsorb onto the catalyst, while the other molecule, nitrogen, exits the catalyst chamber. The catalyst type and residence time can be varied to achieve desired purity levels of the resultant nitrogen stream. However, the PSA process is not preferable because the PSA equipment can be too large and heavy to be easily moved from one location to another. The cost of frequently compressing the air can be prohibitive as well.
The third generally understood method for generating nitrogen is membrane filtration. Membrane filtration is a process in which air passes through a membrane unit which separates some of the oxygen from the nitrogen by means of membrane pores sized to filter the larger molecule, oxygen, out of the smaller molecule, nitrogen. While the membrane quality can be varied to achieve different purity levels of nitrogen, even with the most efficient membranes sufficient oxygen remains in the nitrogen to create corrosion. Thus, the membrane filtration method does not generate nitrogen of sufficient purity to eliminate the need for corrosion inhibitors for under balanced drilling conditions.
The fourth generally understood method for generating nitrogen is combustion. Combustion reactions provide for the burning of a substance in the presence of air to consume the oxygen in the air while leaving the nitrogen intact. One drawback of combustion is that the nitrogen product is mixed with carbon dioxide as a result of the reaction. The combustion reaction can also produce other impurities such as carbon monoxide (CO) and nitrogen oxide (NOX). These pollutants are undesirable in the nitrogen and must be removed in order for the nitrogen stream to be usable for under balanced drilling operations. Therefore, combustion is not an appropriate means for nitrogen production at the well site.
The prior art has previously addressed the need for nitrogen at the well site. For example, U.S. Pat. No. 6,494,262 (the '262 patent) entitled “Non-Cryogenic Production of Nitrogen for On-Site Injection in Well Clean Out” discloses a method for cleaning out a well using a compressed inert gas, such as nitrogen, produced by the non-cryogenic separation of air. The inert gas is delivered to the region of the well where undesirable matter has collected. In particular, the '262 patent provides for the inert gas to be supplied onsite by the separation of air using a membrane filtration or a PSA system. Neither membrane filtration nor PSA can provide the purity level of nitrogen required to eliminate corrosion during under balanced drilling operations. Therefore, a need exists for an improved method for producing nitrogen at a well site that is able to produce nitrogen of sufficient purity to significantly reduce the potential for corrosion in under balanced drilling operations, secondary recovery projects, and pipeline maintenance.
U.S. Pat. No. 6,206,113 (the '113 patent) entitled “Non-Cryogenic Nitrogen for On-Site Downhole Drilling and Post Drilling Operations Apparatus” discloses a method for enhancing hydrocarbon production by delivering a nitrogen rich gas produced from a non-cryogenic source into the well or reservoir where the hydrocarbons are located. In particular, the '113 patent provides for the inert gas, such as nitrogen, to be supplied onsite by separating air using membrane filtration or PSA. Neither membrane filtration nor PSA provides the purity level of nitrogen required to prevent corrosion during under balanced drilling, post drilling operations commonly known as secondary recovery and pipeline maintenance. Therefore, a need exists for an improved method for producing nitrogen at a well or field site that is able to produce nitrogen of sufficient purity to be used in under balanced drilling, secondary recovery, and pipeline maintenance operations.
The four generally understood methods for producing nitrogen are not preferable for under balanced drilling operations. The cost of the cryogenic distillation equipment is prohibitive for under balanced drilling operations. The membrane filtration units typically do not create nitrogen of sufficient purity to prevent corrosion in under balanced drilling operations. The PSA units are bulky and are not sufficiently portable for under balanced drilling operations. The combustion equipment is inexpensive and portable, but produces nitrogen that contains undesirable contaminants rendering the nitrogen unsuitable for under balanced drilling operations. Therefore, a need exists for a method of producing sufficiently pure nitrogen using mobile equipment, in which the nitrogen is suitable for under balanced drilling, secondary recovery, and pipeline maintenance operations.
Recently, a new method of removing oxygen from air, the deoxygenation reaction, has been developed. The deoxygenation reaction uses a platinum catalyst to react the oxygen in air with a hydrogen feed to produce water. The products of the deoxygenation reaction are water and nitrogen. When the correct ratios of air and hydrogen are used, virtually all of the oxygen in the air is converted into water. The resulting nitrogen/water stream can be cooled to condense the water out of the nitrogen, if desired. This process is illustrated in U.S. Pat. No. 6,274,102 (the '102 patent) entitled “Compact Deoxo System.” The deoxygenation reaction in the '102 patent is useful and could be used for under balanced drilling operations. However, because oxygen is a very valuable gas, it would be more desirable for the overall process to separate the oxygen from the nitrogen instead of consuming the oxygen. The oxygen could then be sold to help finance the cost of the deoxygenation equipment, drilling equipment, drilling operations, and production operations. Therefore, a need still exists for a method for separating air into oxygen and nitrogen in which the oxygen stream is not consumed in the process.
Consequently, a need exists for a process to produce nitrogen in which the process equipment can be easily moved to remote locations. The need extends to a nitrogen production method that is able to produce nitrogen of sufficient purity for under balanced drilling, secondary recovery operation, or pipeline maintenance. A need exists for a method of producing nitrogen that is relatively contaminant-free. Finally, a need exists for producing nitrogen in which the oxygen is not consumed in the nitrogen generation process.